Recently, the density of pixels of a camera module, such as a camera mounted in a mobile phone, a digital camera, and the like has been increasing. In regard with camera modules, the miniaturization of pixels is required with an increase in the pixel density. Under this situation, such a problem is at issue that there is an absent portion (hereinafter, appropriately referred to as “a defect”) of a digital image signal due to a pixel which does not normally function. In a defect inspection during the manufacture of a camera module, in the case where pixel defects more than a rule are recognized, the camera module is processed as a defective product. As the rule becomes stricter, the yield of camera modules is reduced and thus the manufacturing cost increases. Accordingly, in the related art, a method of obscuring a defect by signal processing in a defect correction circuit is actively used.
Defect detecting methods are generally classified into two types, a predetection type and a dynamic detection type. The predetection type is a method which detects a defect caused during defect inspection after the manufacture of a camera module and stores address information on the defect in each sensor. The predetection type of method is mainly used for the purpose of correcting a defect caused by a defect of a multi-layer structure, a leakage current of a floating junction, etc. The dynamic detection type is a method which detects a defect from a digital image signal during an operation of a camera module. The dynamic detection type of method is mainly used for the purpose of correcting a photodiode-based defect which randomly occurs depending on a temperature characteristic, an exposure time period, etc.
As a dynamic detection type of defect correction circuit, for example, there is a circuit for performing a defect determination by comparing a difference between a pixel value of a target pixel and the maximum value of pixel values of adjacent pixels with a preset threshold value. Also, there is a circuit for suppressing defect correction on an edge portion of an image to prevent erroneous correction. The noticeability of a defect depends on a luminance distribution of a portion where the defect occurs in an image. For example, a white defect is easily noticeable if existing in a dark portion and a case where erroneous correction has been performed is as easily noticeable as a bright portion. In the case of setting a constant threshold value regardless of luminance, it is difficult to perform defect correction suitably for light and darkness of an image. Further, controlling in order not to perform defect correction on an edge portion has a problem in which correction is not performed even on a noticeable defect existing in the edge portion.